The invention relates to a method for manufacturing thin-walled pipes, which pipes are made of a heat-resistant and wear-resistant aluminum-based material, in particular for use as cylinder liners for internal combustion engines.
Cylinder liners are components subject to wear, which are inserted, pressed or cast into the cylinder openings of the crankcase of the internal combustion engine.
The cylinder faces of an internal combustion engine are subjected to high frictional loads from the pistons or, respectively, from the piston rings and to locally occurring high temperatures. It is therefore necessary that these faces be made of wear-resistant and heat-resistant materials.
In order to achieve this goal, there are numerous processes amongst others to provide the face of the cylinder bore with wear-resistant coatings. Another possibility is to dispose a cylinder liner made of a wear-resistant material in the cylinder. Thus, gray-cast-iron cylinder liners were used, amongst others, which liners however exhibit a low heat conductivity as compared to aluminum-based materials and exhibit other disadvantages.
The problem was first solved with a cast cylinder block made of a hypereutectic aluminum-silicon AlSi alloy. The silicon content is limited to a maximum of 20 weight-percent for reasons associated with casting technology. As a further disadvantage of the casting method it is to be mentioned that primary silicon particles of relatively large dimensions (about 30-80 .mu.m) are precipitated during the solidification of the melt. Based on the size and their angular and sharp-edged form, the primary silicon Si particles lead to wear at the piston and piston rings. One is therefore forced to protect the pistons and the piston rings with corresponding protective layers/coatings. The contact face of the silicon Si particles to the piston/piston ring is flat-smoothed through mechanical machining treatment. An electrochemical treatment then follows to such a mechanical treatment, whereby the aluminum matrix is slightly reset between the silicon Si grains such that the silicon Si grains protrude insignificantly as support structure from the cylinder face. The disadvantage of thus manufactured cylinder barrels lies, on the one hand, in a substantial manufacturing expenditure (costly alloy, expensive mechanical machining treatment, iron-coated pistons, armored and reinforced piston rings) and, on the other hand, in the defective distribution of the primary silicon Si particles. Thus, there are large areas in the microstructure which are free of silicon Si particles and thus are subject to an increased wear. In order to prevent this wear, a relatively thick oil film is required as separation medium between barrel and friction partner. The clearing depth of the silicon Si particles is amongst others decisive for the setting of the oil-film thickness. A relatively thick oil film leads to higher friction losses in the machine and to a larger increase of the pollutant emission.
In comparison, a cylinder block according to the DE 42 30 228, which is cast of an below-eutectic aluminum-silicon AlSi alloy and is provided with liners of a hypereutectic aluminum-silicon AlSi alloy material is more cost advantageous. However, the aforementioned problems are also not solved in this case.
In order to employ the advantages of the hypereutectic aluminum-silicon AlSi alloys as a liner material, the microstructure in regard to the silicon grains is to be changed. As is known, aluminum alloys, which cannot be realized using casting technology, can be custom-produced by powder-metallurgic processes or spray compacting.
Thus, in this way hypereutectic aluminum silicon AlSi alloys are produceable which have a very good wear resistance and receive the required heat resistance through alloying elements such, as for example iron Fe, nickel Ni, or manganese Mn, based on the high silicon content, the fineness of the silicon particles, and the homogeneous distribution. The primary silicon particles present in these alloys have a size of about 0.5 to 20 .mu.m. Therefore, the alloys produced in this way are suited for a liner material.
Even though aluminum alloys are in general easy to be processed, the deformation of these hypereutectic alloys is more problematic. A method for producing liners from a hypereutectic aluminum-silicon alloy is known from the German printed patent document EP 0 635 318. According to this reference the liner is produced by extrusion presses at very high pressures and extrusion rates of from 0.5 to 12 m/min. Very high extrusion rates are required in order to produce the liners to a final dimension with extruders cost-effectively. It has been shown that the high extrusion rates lead to a tearing of the profile during extrusion in case of such difficultly extrudable alloys and of the small wall thicknesses of the liners to be achieved.